Capacitive measurement system

ABSTRACT

Disclosed is a capacitive measurement system. The system comprises structure for generating a plurality of waveforms, a calibrated reference member for use with a reader head in making a measurement, and a reader element configured for lateral movement with respect to the calibrated reference member. The calibrated reference member comprises one element of a capacitor. The reader element comprises a plurality of reader regions comprising the other element of the capacitor. The reader element also includes structure for receiving two signals resulting from coupling of the two capacitor elements. The system functions to detect a phase shift level between the two signals and to determine magnitude of lateral movement in a particular direction from the magnitude of the phase shift.

BACKGROUND OF THE INVENTION

The present invention relates to measurement systems. Although disclosedin the context of a tool the present invention is not limited to suchapplications. Those skilled in the art will recognize that the presentinvention is broadly applicable to measurement systems in general.

SUMMARY OF THE INVENTION

A system is provided for capacitive measurement. The system includesreader reference means comprising a calibrated reference member, readerhead means comprising a reader element, a plurality of reader regions,and signal input means. The system also includes means for generating aplurality of waveforms. Phase shift detection means for detecting apredetermined phase shift level between the two signals received by thesignal input means is also provided. The system determines the magnitudeof the lateral movement of the reader head means with respect to thereader reference means by detecting the magnitude of the phase shift ina particular direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the present invention in use with a tilting arbortable saw.

FIG. 2 illustrates a measurement system reader head and display modulein working relationship with a table saw front guide rail, the Figurealso illustrating a magnetic coupler attached to the table saw fence.

FIG. 3 illustrates a side view of a measurement system display moduleand reader head in working relationship with a cross sectional view of atable saw front guide rail.

FIG. 4 is an exploded view of a display module and reader head.

FIG. 5 illustrates a perspective view of a measurement system readerhead and display module in working relationship with a guide rail.

FIG. 6 is a cross sectional view of a preferred reader head in workingrelationship with a guide rail.

FIG. 7 illustrates a display module remotely coupled to a reader head.

FIG. 8 illustrates a measurement system in a vertical orientation inworking relationship with a thickness planer, the Figure alsoillustrating a magnetic coupler attached to the table of the thicknessplaner.

FIG. 9 is a block diagram of the present measurement system.

FIG. 10 illustrates a reader element.

FIG. 11 illustrates a reader strip or calibrated reference memberforming a portion of a preferred reader reference means.

FIG. 11a is a cross sectional view of a preferred reader strip orcalibrated reference member.

FIGS. 12-15 illustrate preferred electronics for use in connection withthe present system.

FIG. 16-19 illustrate preferred software for use with the presentsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed preferred embodiments of the present invention are disclosed.It is to be understood, however, that the disclosed embodiments aremerely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedare not to be interpreted as limiting, but rather as a representativebasis for teaching one skilled in the art to variously employ thepresent invention in virtually any appropriately detailed system orstructure. It will be understood that in some circumstances relativematerial thicknesses and relative component sizes may be shownexaggerated to facilitate an understanding of the invention.

The present invention relates to an electronic measurement system whichhas the flexibility to be used in a great variety of systems andapparatus. The present invention has several unique preferred featuresincluding a two piece reader head 100 and display module 102, which canbe assembled for vertical, horizontal or remote applications, examplesof which are shown in FIGS. 8, 1, and 7, respectively.

As illustrated in FIG. 1, the tracking design of preferred reader head100 is configured to ride directly on a Unifence®saw guide 106 frontguide rail 104 manufactured by Delta International Corp., 246 AlphaDrive, Pittsburgh, Pa., 15238. Unifence® saw guide 106 is described inmore detail in U.S. Pat. 4,600,184. Alternately, preferred reader head100 can be retrofitted to virtually any tool or other system requiring ameasurement through the use of an auxiliary guide rail such as rail 108shown in FIG. 8. As illustrated, auxiliary guide rail 108 is in avertical orientation for use with a thickness planer 110. Auxiliaryguide rail 108 can be obtained or fabricated in any length desired andcan be installed in any orientation. It can also be retrofitted tovirtually any existing system, including a table saw having an existingfront guide rail for use with an existing saw guide or fence, to a drillpress, to a shaper, or to many other tools or systems requiring ameasurement system.

A preferred measurement system shown in FIG. 2, incorporates magneticcoupler means 112 comprising a magnet 114 for coupling reader head 100to a physical structure such as fence 106, planer table 116, FIG. 8, orother structure by magnetic attraction between the magnet, the readerhead, and the structure.

In the preferred embodiment, shown in FIGS. 2-6, reader head 100comprises a ferrous surface formed on each side of the reader head inthe form of metal side plates 118, while magnetic coupler means 112comprises a bracket 120 for connecting magnet 114 to physical structuresuch as saw fence 106, planer table 116, or other structure. Thisembodiment permits use of the present system with physical structurethat does not already include ferrous material. Alternately, reader head100 may comprise magnetic side plates such as 118, which can beintegrally formed with reader head 100 and directly coupled to aphysical structure comprising a ferrous material.

Magnetic coupler means 112 has many advantages, including accurate andrepeatable coupling and positioning of the reader head with physicalstructures to be tracked for measurement location, while at the sametime forming not only an easy disconnect means but also means forpermitting the connection between reader head 100 and the physicalstructure to flex, e.g., during adjustments or removal of a fence suchas 106 or during adjustment or removal of other physical structurerequiring coupling with a measurement system. For example, in connectionwith a saw fence application, magnetic coupler means 112 can be used toconnect the reader head/display module assembly to the moveable portionof the fence that travels along a front guide rail. In this application,magnetic coupler means 112 ensures precise positive alignment in thedirection of travel, and allows movement and misalignment in the othertwo directions. This enables a table saw fence measurement system to beaccurate, and it allows easy removal of the fence from the saw.

Preferred auxiliary guide rail 108 has a cross-sectional configurationsimilar to the geometry a Unifence® saw guide 106 front guide rail 104.As previously indicated, auxiliary guide rail 108 may be mounted eitherhorizontally, vertically or in some other orientation, in a variety ofapplications. For example, a long length of auxiliary guide rail 108 canbe integrated with an existing generic table saw front guide rail andsaw fence in order to replace or enhance an analog mechanicalmeasurement system. Those skilled in the art will recognize that ananalog mechanical measurement system having a human-readable graduatedscale and a mechanical indicator for use in reading the graduated scaleis subject to human error due to the interpretation required in readingthe system. As will be further described below, the present electronicmeasurement system eliminates this human error by providing extremelyprecise, direct measurements. Further, through the use of an auxiliaryguide rail, such as 108, for guiding the lateral movement of reader head100 and through the use of magnetic coupler means 112 for coupling thereader head with physical structure such as an existing tool fence, anexisting analog mechanical measurement system can easily be replaced orenhanced.

In operation, guide rails such as 104 and 108 form a reader referencemeans comprising a reader strip or calibrated reference member such as202, shown in FIG. 11, for use with reader head 100 in making ameasurement. In the preferred embodiment, reader strip or calibratedreference member 202 comprises metalized signal regions 262 which formone element of a capacitor. Further, reader head 100 comprises a readerelement such as 204, shown in FIGS. 10 and 12, which is preferablyconfigured to move laterally with respect to reader strip or calibratedreference member 202. In the preferred embodiment, reader element 204comprises metalized reader regions such as reader regions 280 through295, which comprise the other element of the capacitor. The manner inwhich calibrated reference member 202 and reader element 204 interact toprovide a measurement will be described below.

FIG. 11a illustrates a preferred embodiment reader strip or calibratedreference member 202, preferably comprising a substrate layer 271 whichprovides a basic physical support structure for reader strip orcalibrated reference member 202. Reader strip or calibrated referencemember 202 further comprises a layer of pressure sensitive adhesive 272attached to the surface opposite the surface carrying metalized signalregions 262. A removable protective covering 274 is placed over thelayer of pressure sensitive adhesive 272 for protecting the adhesiveuntil the time that the reader strip or calibrated reference member isto be installed.

Preferred reader strip or calibrated reference member 202 comprises ametalized pattern on the surface opposite the adhesive 272 surface. Asshown in FIG. 11, the pattern comprises a metalized ground strip 260 anda plurality of metalized ground regions 260a, each having apredetermined width and parallel sides oriented perpendicular to thelength of metalized ground strip 260. Metalized ground regions 260a arespaced a predetermined distance from one another and are conductivelycoupled to metalized ground strip 260.

As previously indicated, the metalized pattern of preferred reader stripor calibrated reference member 202 further comprises metalized signalregions 262 alternately located between metalized ground regions 260a.Metalized signal regions 262 also have a predetermined width andparallel sides oriented perpendicular to the length of metalized groundstrip 260. Signal regions 262 are spaced a predetermined distance fromone another and from metalized ground regions 260a, with metalizedsignal regions 262 being electrically isolated, both from one anotherand from metalized ground strip 260. In a preferred embodiment, themetalized pattern comprises nickel plated copper and a protectivedielectric layer 270 which covers the metalized ground and signalregions.

Preferred metalized reader regions such as reader regions 280 through295 illustrated in FIG. 10 also have a predetermined width and parallelsides. Similarly, the reader regions are spaced a predetermined distancefrom one another, the reader regions also being electrically isolatedfrom one another.

In operation with the present system, preferred adhesive backed readerstrip or calibrated reference member 202 is attached to a guide railsuch as 104 or 108, and reader element 204 is secured in reader head 100so that the sides of signal regions 262 are parallel to the sides of thereader regions such as reader regions 280 through 295, with readerelement 204 and its corresponding reader regions moving laterally withrespect to reader strip 202 and its corresponding signal regions 262.

The most critical area of an electronic measurement system, whethercapacitive, magnetic, optical, or resistive, is frequently how thereader head components, particularly reader element components, maintaina precise relationship to a reader strip or calibrated reference memberin order to give a high degree of resolution and accuracy. For example,in typical prior art electronic caliper capacitive measurement systems,and in electronic measurement systems such as those disclosed in U.S.Pat. No. 4,694,275, it is extremely important that capacitive readerelements are in substantially intimate contact with capacitive signalelements, the two elements typically being separated only by a thinlayer of dielectric. In the system disclosed in U.S. Pat. No. 4,694,275,for example, the capacitive reader element and the capacitive signalelement are biased together in a rubbing relationship by using a foambias, with the two capacitive elements being separated only by a thindielectric layer. Unfortunately, this dielectric layer is subject towear. In alternate approaches, such as those typically used in preciselymachined caliper systems, the precise relationship required between thecapacitive reader elements and the capacitive signal elements ismaintained through precision machining of the corresponding reader headand guide rail.

The electronics of the present system permit a variable standoff to beemployed between reader element 204 and reader strip or calibratedreference member 202 while still maintaining extremely precisemeasurements. Accordingly, it has been found that the necessaryprecision in defining the physical relationship between reader element204 and reader strip or calibrated reference member 202 can beaccomplished through preferred standoff means for establishing avariable standoff distance between the reader element and the readerstrip or calibrated reference member while substantially invariablymaintaining a fixed lateral position of the reader element within thereader head.

In the preferred system, this is accomplished by a unique andinexpensive approach which permits use of a circuit board to form thebody of reader element 204, which permits the use of an inexpensiveextrusion for forming the body of reader module 100, and which permitsuse of inexpensive extruded material for guide rails such as 104 or 108.Additional components used in the preferred system to accomplish this,which are shown in FIG. 6, include two wiper springs 122, one wiperstandoff mechanism 123, preferably formed of plastic, and twocompression springs 124. Wiper springs 122 are preferably bonded toreader element 204 and are used in the preferred embodiment to performseveral functions. First, as can be seen in FIG. 6, leaf spring sections126 are deformed toward each other and maintain a preload when thereader element/leaf spring assembly is inserted into reader head 100.This preload prohibits reader element 204 from moving laterally withrespect to the reader module. Second, a small wiper lip 128 on eachwiper spring 122 spaces reader element 204 at a nominally constantstandoff distance from the surface of reader strip or calibratedreference member 202. In addition, wiper standoff mechanism 123 pushesreader element 204 away from reader strip 202 and wipes reader strip 202clean of dust and debris. Further compression springs 124 push wipersprings 122 against reader strip 202. The combination of wiper standoffmechanism 123 and compression springs 124 cause each wiper lip 128 tomaintain continuous contact with reader strip 202 and to wipe it cleanof dust and debris.

Preferably, spring forces and materials are precisely matched to insurethat the wiper spring/circuit board assembly can float up and down inthe direction of compression springs 124 while at the same time insuringthat there is no lateral movement of reader element 204 in reader headmodule 100 as module 100 is moved back and forth along reader strip orcalibrated reference member 202. Using this approach, all components maybe inexpensively manufactured using standard liberal tolerances, whilestill maintaining a high degree of resolution and accuracy, which isonly maintained in other designs by extremely strict tolerances andprecision machining or by a rubbing relationship between the reader andreader strip elements, such as in the system disclosed in U.S. Pat. No.4,694,275.

As previously indicated, in the preferred embodiment, signal regions 260of reader strip or calibrated reference member 202 comprises one elementof a capacitor, and reader element 204 comprises the other element ofthe capacitor. However, the present standoff system could apply equallywell to use with optical, resistive, or magnetic measurement systemswhere precise lateral positioning of the reader element within a readerhead is desired, while at the same time permitting a variable standoffto occur whenever perturbations such as those encountered withinexpensive guide rail extrusions are encountered

As has previously been indicated, the present system has multiple readerhead display orientations. Reader head 100 and display module 102 may beconnected so that display means 210 can be read horizontally independentof whether a guide rail such as 104 or 108 or other reader referencemeans is oriented horizontally (as in FIGS. 1), vertically (as in FIG.8) or in some other direction. Horizontal orientation of display 210, asshown in FIG. 7, in connection with virtually any orientation of a guiderail or other reader reference or in connection with a remote locationof display module 102 from reader head 100 can be accomplished throughremote connection means 142. As is illustrated in FIG. 7, remoteconnection means 142 may comprise a cord connection for remotelyconnecting display module 102 to reader head 100. In the preferredsystem, all installations, whether horizontal, vertical, or remote, usethe same hardware, with remote connection means 142 being used forremote installation of display module 102.

Examples of using a remote location with display module 102 include useof the present system with a shaper, with reader head 100 typicallybeing coupled with a vertical guide track such as 108 to the spindleelevation system below the shaper table, and with display module 102being located in a convenient location on the exterior of the shaper,typically somewhere above the shaper table.

Based on present plans, various elements of the present system will beavailable from Delta International Corp. in standard kits as follows. Akit for use in retrofitting the present system to a Unisaw® tiltingarbor saw or other table saw having a Unifence® saw guide will beavailable under part no. 32-010 and will include preferred reader head100, display module 102, preferred reader strip or calibrated referencemember 202, and hardware with which to configure magnetic coupler means112. A vertical kit comprising a relatively short guide rail such as108, reader head 100, display module 102 and remote connection means142, and reader strip or calibrated reference member 202 will beprovided under part no. 32-011. Preferred reader strip or calibratedreference member 202 will be available under part no. 32-012, and a longlength of guide rail such as 108 together with miscellaneous mountinghardware will be available under part no. 32-013.

Vertical kit 32-011 will also include adjustment hardware 130 comprisinga slotted bracket 134 for receiving guide rail 108 and for adjusting theelevation of guide rail 108 through the use of a threaded adjustmentknob 132. Adjustment hardware 130 further typically comprises lockingmeans 136 for locking guide rail 108 into a fixed location following itsvertical height adjustment. Locking means 136 may comprise threadedlocking bolts which pull guide rail 108 tightly against one surface ofthe slot in bracket 134. This is a particularly important feature whenusing a vertical kit with a tool such as a planer where the planercutting blades may be below the upper surface of a work bench or table.The vertical kit would thus combine the mechanical and electronicadjustments.

In using the present system, illustrated in block diagram form in FIG.9, the preferred system software waits after power-up for the user topress a zero key 212a before producing a display. For use with a tablesaw, as depicted in FIG. 1, the user would typically slide fence 106down guide rail 104 until fence 106 reaches the saw blade. To establishthis position as the reference point, the user presses zero key 212a.The display will then read "0.00" in any of the available measurementunits. The preferred software is then ready to accept further keyinputs.

To select preferred directional options available throughout the presentsystem, e.g., right-of-blade or left-of-blade (ROB/LOB), the userpresses zero key 212a as described in Table 1 below. When the desiredoption is obtained, the zero key is released. In the "right-of-blade"option, movement to the right is considered positive, while movement tothe left is considered negative. In the "left-of-blade" option, thesepolarities are reversed. In the preferred embodiment, this option isdenoted by an "L" on the left of display means 210.

Accordingly, the present system comprises first mode display means forincreasing the measurement as reader element 204 moves away from a firstside of a reference position and for decreasing the measurement asreader element 204 moves toward the first side of the referenceposition. The present system further comprises second mode display meansfor increasing the measurement as the reader element moves away from theother side of the reference position and for decreasing the measurementas the reader element moves toward the other side of the referenceposition. In a table saw environment, such as FIG. 1, these modes ofoperation comprise right-of-blade mode display means for increasing themeasurement as saw fence 106 moves away from the right side of the sawblade and for decreasing the measurement as saw fence 106 moves towardthe right side of the saw blade. In this configuration, the presentsystem further comprises left-of-blade mode display means for increasingthe measurement as saw fence 106 moves away from the left side of blade211 and for decreasing the measurement as fence 106 moves toward theright side of the saw blade.

To select the desired precision, or resolution, options, the userpresses mode key 212b as described in Table 1. When the desired optionis obtained, the mode key is released. In the preferred embodiment, alow resolution option provides measurements in increments of 0.005inches or 0.1 millimeters, and a high resolution option providesmeasurements in increments of 0.001 inches or 0.01 millimeters. Inpreferred display means 210, the high resolution option is denoted by"HR" on the left of the display. The low resolution option is more thanadequate for table saws, while the high resolution option may berequired for other applications, such as thickness planers. Neitheroption affects a fractional mode, which in the preferred embodimentalways provides measurements in increments of 1/32 inch.

Accordingly, the present system comprises means for displaying ameasurement in both decimal and fractional units, for displaying themeasurement in both English and metric units, for establishing a zeroreading at any location of reader head 100, and for providing themeasurement in either of two ranges of precision, e.g. high or lowresolution.

Once the options have been selected after power up and the presentsystem has been "zeroed" to a blade or other reference position, thepresent system is ready for use as an accurate device for measuringdistance from the reference position. After work is done and the presentsystem is idle, the user may turn off display 210 in order to prolongbattery life. Although this is not necessary, it is provided as anoption to the occasional user, such as a home shop craftsman who doesnot use a saw as often as, for example, a cabinet maker. In thepreferred embodiment, the standby option is selected when the user firstpresses and holds zero key 212a then quickly presses the mode key withinone second. Preferred display 210 then goes blank, and the softwarestops executing. Table 1 provides a summary of keys and relatedfunctions which comprises the human interface of the preferredelectronic measurement system (EMS).

                  TABLE 1                                                         ______________________________________                                        Summary of Keys                                                               ______________________________________                                        ZERO                                                                          Pressed once:                                                                 "Wakes-up"EMS from Standby Mode, turns display                                210 on, resets EMS.                                                           Sets current fence position as new reference                                  point (zero).                                                                 Pressed and Held Two (2) Seconds:                                             Toggles EMS between Left-of-Blade,                                            Right-of-Blade Option. Changes every 2                                        seconds if held down.                                                         MODE                                                                          Pressed once:                                                                 Toggles display 210 through Inch/Millimeter/                                  Fraction Modes.                                                               Pressed and Held Two (2) Seconds:                                             Toggles EMS between High Resolution/Low                                       Resolution option. Changes every 2                                            seconds if held down.                                                         ZERO + MODE                                                                   In the preferred embodiment, done in this sequence:                           Press and Hold ZERO then quickly press MODE.                                  This puts EMS in Stand-by Mode and blanks                                     display 210. Used when EMS will be idle for                                   a long period of time.                                                        ______________________________________                                    

Note that, in woodworking and other applications, the present systemprovides the ability to retrofit a measurement system to a device thatis presently relatively imprecise in order to make the device performwith extremely high precision. In addition, it can be used to create newdesigns of such devices having extremely high precision, and it can beused to increase further the precision of what are presently relativelyprecise devices. For example, by providing a guide rail such as 108 inconjunction with the cross cutting fence of a radial arm saw or powermiter box, and by using a stop member in conjunction with reader head100/display module 102, a "zero" length for a cross cut can easily beset by referencing off of a blade tooth, and the exact length of cutdesired can be made the first time by moving reader head 100 to thedesired location, reading the exact dimension off of display module 102,and using the stop member as a guide for positioning the workpiecebefore making the cut.

Further, such a measurement for cutting a workpiece can be accomplishedwithout human interpretation of a mechanical analog scale. In addition,the exact workpiece dimension can be obtained in one try, providing ahuge increase in productivity and savings in material cost, since wastedeffort and materials due to inaccurate cuts can be eliminated.

Further, the exact dimension of a workpiece can be established, not onlywithout human interpretation of an analog scale, but the dimension canbe made and displayed in various formats, such as in either decimal orfractional form and in either English or metric units. The use ofdecimal units permits use of conventionally available high precisionmeasurement systems such as calipers to measure the desired dimension ofa workpiece (e.g., to fit into a known location in a carcase) and toduplicate the exact dimension by using decimal units in display module102. Thus, no unit conversion table or separate calculation isnecessary. On the other hand, use of fractional units such as the 1/32inch increments available through the preferred system permits awoodworker accustomed to English fractional units to use them.Alternately, use of metric units permits working directly to Europeanmetric standards. Again, all of these selections can be made and usedwithout the human interpretation required to read mechanical analogscales, and without the need to manually convert to the desired units ofmeasurement.

Note further that, through the use of a standard configuration guiderail such as 108 on all tools or other devices where increasedmeasurement precision is desired and, through the use of magneticcoupling means 112, the occasional user can quickly and easily share asingle reader head 100/readout module 102 among a large variety ofdevices, thus greatly enhancing productivity and precision at arelatively low cost.

Referring now to FIG. 9, as previously indicated the components ofpreferred electronic measurement system (EMS) 200 combine to provide theuser with an accurate digital representation of linear movementdisplayed in English or metric units in both directions along a singleaxis, which may be oriented vertically, horizontally or in some otherdirection. EMS 200 comprises reader reference means 202, reader element204, motion detection means 206, logic means 208, display means 210 anduser keypad 212 comprising keys 212a and 212b.

An embodiment of reader head 100 comprising reader element 204 is shownin FIG. 12. FIG. 12 shows a plurality of preferred reader regions 280through 295 which form one element of a capacitor with preferredmetalized signal regions 262 (See FIG. 11) forming the other element ofthe capacitor. In the preferred embodiment, reader regions 280 through287 form a group of eight reader regions which receive eight differentwaveforms. Each group of eight reader regions in reader element 204 areelectrically coupled together through electric couplers 296, 298, 300,302, 304, 306, 308 and 310. For example, reader region 280 is coupled toreader region 288 through electric coupler 296. Reader region 281 iscoupled to reader region 289 through electric coupler 298. Reader region282 is coupled to reader region 290 through electric coupler 300. Readerregion 283 is coupled to reader region 291 through electric coupler 302.Reader region 284 is coupled to reader region 292 through electriccoupler 304. Reader region 285 is coupled to reader region 293 throughelectric coupler 306. Reader region 286 is coupled to reader region 294through electric coupler 308. Finally, reader region 287 is coupled toreader region 295 through electric coupler 310. As a result, readerregion group 280 through 287 receives the same waveforms as readerregion group 288 through 295. Similarly, the other two reader regiongroups receive the same waveforms. Thus, each of eight differentwaveforms are produced in four locations along reader element 204.

Preferred reader element 204 further comprises signal input means 250for receiving two signals resulting from coupling of the two capacitorelements, the metalized signal regions 262 and the reader regions ofreader element 204 in combination with the eight different waveformscoupled between them. Signal input means 250 is preferably coupled toelectric coupler 252 to transfer the received signals to other elementsof the preferred system 200 to be described later. Signal input means250 further is electrically isolated from the reader regions of readerelement 204 by a grounded region 254 electrically depicted in FIG. 12and physically depicted in FIGS. 9 and 10. Grounded region 254 preventsdirect coupling of the eight different waveforms received by the readerregions to signal input means 250. This configuration provides a signalinterlock whereby reader element 204 and calibrated reference member 202will produce no signal unless the two capacitive elements are inparallel alignment and close physical proximity. This elementarrangement insures that only lateral movement of reader element 204will result in motion detection. Further, this preferred capacitivearrangement will tolerate some amount of "twisting" and "air-gap"between reader element 204 and calibrated reference member 202 withoutaccumulation of a waveform resulting in measurement error. Thus, onlyindirect coupling of the waveforms through metalized signal region 262is allowed.

Referring to FIGS. 11 and 12, when waveforms are applied to readerelement 204 through electric couples 296, 298, 302, 304, 306, 308 and310, reader element 204 induces signals on calibrated reference member202 due to the capacitance developed by the close proximity of the twoconductive elements, i.e., metalized signal regions 262 and the readerregions of reader element 204. These signals are then picked up byreader element 204 and signal input means 250, the latter also beingknown as a "wiper". Metalized signal regions 262 allow the accumulationof charge on their surfaces while the metalized ground regions 260 and260a interrupt the charge pattern between the various metalized signalregions 262. As reader element 204 is moved, the reader regions meetalternately with metalized signal regions 262 and metalized groundregions 260 on calibrated reference member 202, thus causing a dynamiccapacitance change. The resulting signal waveform from this rathercomplex combination of different waveforms, wave shifts and dynamiccapacitance i a signal picked up by signal input means 250; in thepreferred system, this signal appears as two approximately identicalwaveforms which are 180° out of phase when reader element 204 is atrest. The phase shift is caused by the alternating metalized signalregions 262 and the metalized ground regions 260a. When reader element262 moves in a lateral direction, the phase shift changes. It is thephase shift modulation which produces a varying signal picked up bysignal input means 262 and which indicates motion.

It can be recognized that different waveforms will be induced onmetalized signal regions 262 and then will be induced on metalizedground regions 260a, that each group of reader regions will have acertain number of reader regions near metalized signal regions 262 andthat some reader regions will be over metalized signal regions 260a.Depending on the direction--in a first direction or an opposite seconddirection--of movement, the number of reader regions over the metalizedsignal regions 262 will decrease or increase. In the preferredembodiment, if the number of reader regions over metalized signalregions 262 decrease, the phase shift will drop below 180°; if thenumber of reader regions over metalized signal regions 262 increase, thephase shift will rise above 180°. This provides a method of detectingthe direction of travel of reader element 204 which can be recognized byother elements of EMS 200 to be described later. Both the magnitude andpolarity of linear motion are detected by monitoring only the phaseshift of the signal picked up by signal input means 250.

Since the phase shift is independent (to a certain degree) of themagnitude, and even the frequency, of the waveforms carried by thereader regions, the signal quality is highly immune from power supplyvariation, input noise, frequency variation and slight misalignment.This independence is because such perturbations would appear common toboth of the signal waveforms picked up by signal input means 250. Sincethe preferred system measures only the difference in phase of the twowaveforms in the signal, such "common mode" variations are ignored. Thisphenomenon is also known common-mode rejection.

Therefore, due to the configuration and operation of reader strip orcalibrated reference member 202 in combination with reader element 204,the present system will give very accurate signals to signal input means250 with a high degree of noise immunity and false motion signalrejection. For instance, bumping a table saw arrangement with theelectric measurement system attached or starting the saw motor will notcause a change in the signal waveform received by signal input means250.

FIG. 13 shows motion detection means 206. In the preferred embodiment, amicrochip known as SNC5125 available from Excel Technology InternationalCorporation of Belmead, N.J. is used to provide many of the necessaryfunctions for motion detections means 206. Motion detection means 206comprises means for generating a plurality of analog waveforms.

The generation of analog waveforms is done within microchip 320. Thegeneration of waveforms are output through electric couplers 296, 298,300, 302, 304, 306, 308 and 310. The output waveforms are typically ofthe same shape and frequency but with different phase angles. The phasedifference of the waveforms are proportionately larger between thewaveforms carried by electric couplers 296, 298, 300, 302, 304, 306, 308and 310 with electric coupler 310 carrying a waveform with the largestphase difference with respect to the waveform carried by electriccoupler 296. Microchip 320 further comprises oscillating means forgenerating a clock signal. The frequency of the oscillating means is setby resistive element 322. Microchip 320 outputs the clock signal throughelectric coupler 358. Resistive element 324 sets the sensitivity of theamplifier input of microchip 320 coupled to electric coupler 252. Bychoosing a relatively large resistance in resistive element 324, highfrequencies are filtered out of the signal coupled to the amplifierinput of microchip 320 through electric coupler 252. Resistive element330, as well as capacitive elements 326, 328 and 332, form a wave shapernetwork 334 which helps convert the analog signal received by theamplifier input of microchip 320 into a digital signal. Microchip 320further comprises means for further converting the received analogsignal into a digital signal. Microchip 230 further comprises a digitalcounter for storing the position of reader element 204 with respect tocalibrated reference member 202. A reset input is coupled to electriccoupler 340. Power is supplied to microchip 320 through electric coupler362. An electrical ground is supplied to microchip 230 through electriccoupler 362. Microchip 230 latch input is coupled to electric coupler356. Microchip 230 also contains "hand shake" inputs which are coupledto electric couples 350, 352 and 354, respectively. Microchip 230further comprises counter outputs which are coupled to electric couples342, 344, 346 and 348, respectively.

In operation, motion detection means 206, also know as phase shiftdetection means 206, converts the analog signal received from electricalcoupler 252 to a digital signal. The digital signal is indicative of thephase shift variance from 180° phase separation of the two waveformspresent in the received signal. The digital signal is a pulse string,the duration of which is determined by the magnitude of the variance.Phase shift detection means 206 also produces an increment/decrementsignal indicative of which mode of operation the counter should be in.The pulse string is fed into the digital counter. The digital counterwill increment during the time period that the pulse string is a logical"high" and the increment/decrement signal is a logical "high". Duringthis time period, the digital counter may increment a plurality of timesif the duration of the pulse string logical "high" is long enough.Similarly, the digital counter will decrement during the time periodthat the pulse string is a logical "high" and the increment/decrementsignal is a logical "low". During this time period, the digital countermay decrement a plurality of times if the duration of the pulse stringlogical "high" is long enough.

FIG. 9 shows logic means 208 and keypad means 212. Keypad means 212comprises key 212a and key means 212b. As has previously been indicated(See Table 1 and accompanying text) key 212a is activated by an operatorto zero the digital counter of motion detection means 206 or,alternatively, to toggle the direction of movement from a referencepoint such as "left-of-blade" or "right-of-blade" Key 212b is activatedby an operator to toggle display modes of EMS 200 or, alternatively, totoggle the display precision of a particular readout.

Logic means 208 controls the operation of preferred EMS 200. In thepreferred embodiment, a microchip, known as LC5851, from Tokyo SanyoElectric Company, Ltd., is used to provide many of the necessaryfunctions for logic means 208. Electric coupler 362 is coupled to apower supply 378. In a preferred embodiment, two size "AA" batteries,housed in battery compartment 103 behind battery door 105 (See FIG. 4),are used to supply power for EMS 200. Power supply 378 is also coupledto a power input of microchip 370 and key 212a. A decoupling capacitoris coupled between power supply 378 and another power input of microchip370. Capacitor 374 sets the bias for an LCD readout 210 coupled throughmicrochip 370 output couplers 380 through 405.

A small backup power supply preferably is maintained by capacitor 376.Capacitor 376 will retain the state of digital memory during shortperiods of power interruption. Microchip 370 comprises inputs 9, 10, 11and 12 which are coupled to an electrical ground to help minimize noiseinput in EMS 200. Electric coupler 340 couples key 212a and a resetinput of microchip 370. When key 212a is activated by an operator, areset signal is sent to microchip 370 and to the electric coupler 340.Electric couplers 342, 344, 346 and 348 are coupled to data inputs onmicrochip 370. Electric couplers 350, 352, 354, and 356 coupled todigital "hand shake" outputs of microchip 370. Electric coupler 358couplers to a clock input of microchip 370. The signal provided byelectric coupler 358 determines the operational frequency of EMS 200.Electric coupler 360 supplies an electrical ground to microchip 370 andone input of backup power supply 376. Electric coupler 362 couplers keymeans 212b and power supply 378. When key means 212b is activated by anoperator a toggle mode signal is sent to microchip 370. Electriccouplers 380, 381, and 382 coupler microchip 370 outputs to back planeinputs of a display means 210 shown in FIG. 15. In a preferredembodiment display means 210 comprises a liquid crystal display (LCD)210 readout device such as an Excelix 7304 LCD. Electric couplers 383through 405 coupler microchip 370, outputs to the front plane inputs ofdisplay means 210 shown in FIG. 15. Electric coupler 406 couplers asignal from microchip 370, which sets the voltage level of display 210.

Logic means 208 controls the operation of EMS 200. Software embedded inlogic means 208 sends out "hand shake" signals to motion detection means206 which cause the digital counter to output data to logic means 208.Logic means 208 also has the capability of resetting the digital counterof motion detector means 206 to a zero state. The resetting allows theoperator to define a reference point anywhere along reader strip orcalibrated reference member 202. Due to the choice of particular devicesin the preferred embodiment, EMS 200 is capable of accurately measuringlinear motion at speeds in excess of 36 inches per second with an errorof +/-0.001 inch.

Digital data provided by the preferred electronics is formulated bysoftware in logic means 208 into English or metric units of measurementand then displayed on display means 210. Description of a preferredembodiment of the embedded software follows.

Referring now to FIG. 16, after power up the software initializes thesystem 402. Certain memory locations are initialized, display 210 isblanked and a standby mode 402 is entered. During standby mode 402 EMSsystem 200 consumes very little power. To "wake up" EMS system 200, zerokey 212a must be pressed. The software looks for zero key 212a beingpressed at 406. If zero key 212a is not pressed the software returns tostandby mode 404 and periodically checks for zero key 212a being pressed406. Once zero key 212a is pressed 406, the software turns on thedisplay at 408 and goes to a main loop 412.

Referring now to FIG. 17, main loop 412 is a routine that monitors keypad 212 and updates display 210. If zero key 212a and mode key 212b arepressed 414, then the software returns to standby mode 416. Otherwise,the software gets the counter value 418. After getting the countervalue, the software determines if the counter is counting up or countingdown 420. If it is counting up, the direction of movement is positive.If it is counting down, then the direction is negative. In a preferredembodiment if EMS 200 is set in a "left-of-blade" option, the softwarecomplements the value of the counter and then determines the directionof movement to be positive or negative. This preferred embodiment"left-of-blade" option is most useful in a table saw or similarenvironment. Otherwise, if the software is set in a "right-of-blade"option, the software does not complement the value of the counter. Thesoftware also determines if an overflow condition has occurred 420. Ifan overflow condition has occurred, an overflow register is incremented.After determining direction of movement and overflow condition, thesoftware looks for depression of user keys 212; if no user key isdepressed then the software goes to routine up data display 424. If akey is pressed, then the software determines which key 212a or 212b waspressed 426. If zero key 212a was pressed the software resets thereference point 430 by resetting the digital counter of motion detectionmeans 206 and clearing display means 210. The software then checks tosee if zero key 212a has been held down for two seconds 434. If it hasnot been held down for two seconds then the software goes to updatedisplay subroutine 442. Otherwise if the zero key was held down for twoseconds the software toggles the direction mode options 438 of EMSsystem 200. In the preferred embodiment, if a "left-of-blade" mode ofoperation was in effect then the software is toggled to a"right-of-blade" mode of operation. Otherwise if a "right-of-blade" modeof operation was active then the software is toggled to a"left-of-blade" mode of operation. Once the software has been toggledbetween different directions, the software goes to update displaysubroutine 442.

If the mode key was pressed 426, then the software enters a toggledisplay modes option 428. During the toggle display modes 428 operationthe software toggles from one set of units of measurement to the next.In the preferred embodiment the software toggles between metric, Englishdecimal and English fractional units of measurement. Optionally, thesoftware outputs the current position of the device 432 to a serial porton logic means 210. Otherwise the software checks for mode key 212bbeing pressed for two seconds. If the mode key is not depressed for twoseconds then the software goes to subroutine update display 444.Otherwise if the mode key is depressed for two seconds, the softwaretoggles between different display precisions 440. In the preferredembodiment two ranges of precision are used. They are termed lowprecision or resolution and high precision or resolution respectively.If the software was running in low precision then the software changesto high precision mode of operation. If the software was running in highprecision mode of operation then the software changes to low precisionmode of operation. The software then goes to subroutine update display444.

In the preferred embodiment two different resolution microchips 320 areused. Upon entering update display subroutine 446, the softwaredetermines which resolution microchip 320 is being used 448, as shown inFIG. 18. If the low resolution microchip 320 is not used, the softwarechecks which display mode is in operation 452. Otherwise, if a lowresolution microchip 320 is being used by EMS system 200 the softwaredoubles the counter value 450 and checks which display mode is inoperation 452.

If the display mode in operation is an English display mode, then thesoftware converts the counter value to an English decimal value 454. Thesystem then determines if the display mode is fractional or decimal 456.If the fractional mode of operation is currently set, then the softwareconverts the English decimal counter value to a fractional value 458 anddoes a subroutine update display of fractions 462. During subroutineupdate display fractions subroutine 462 display means 210 is updated toshow an English fractional units. Further, the English fractionalposition value displayed by display 210 is the lowest commondenomination (LCD); for instance, if the fractional value is 12 16/32then the display means displays 12 1/2. Otherwise, if the display modeis a decimal display, the software determines if the change in thecounter value is greater than the precision setting of the software 460.If the change in counter value is smaller than the precision setting ofthe software then the software checks to see if the counter value waspreviously doubled during this main loop sequence 472. Otherwise if thechange in counter value is greater than the precision setting of thesoftware, an update display inches subroutine 464 is entered. Duringupdate display inches subroutine 464, display means 210 is updated toslow the present position of reader head means 204 position relative toa reference part sent by the operation in English decimal units. Thesoftware then checks to see if the counter value was previously doubled472 during this main loop sequence.

If the software is in a metric display mode 452 then the softwareconverts the counter value to a metric value 466. The software thendetermines if the change between this counter value and the previouscounter value is greater than the precision setting of the software 468.If the change in counter value is not greater than the precision settingof the software, then the software checks to see if the counter valuewas previously doubled 472. Otherwise if the change in counter value isgreater than the precision setting of the software, the software entersa subroutine update display millimeters 470. During update displaymillimeters subroutine 470 display means 210 is updated to show thepresent position of reader element 204 relative to a reference point setby the operator in millimeters. Then, the software checks to see if thecounter value was previously doubled 472.

If the counter value was not previously doubled, the software goes tothe main loop subroutine and starts a new sequence 476. Otherwise,software divides the counter value by two 474 and goes to main loopsubroutine 476 and starts a new main loop sequence.

In addition to the linear measurements discussed thus far, the presentsystem can easily be configured to generate highly precise angularmeasurements, thus enabling the development of devices havingsubstantially enhanced precision with respect to angular settings.Examples in the woodworking industry include radial arm saw angularsettings and tilting arbor table saw angular settings havingsubstantially enhanced accuracy over those disclosed in U.S. Pat. No.4,641,557 assigned to Emerson Electric.

Further, in addition to having substantially enhanced precision over thetools disclosed in U.S. Pat. No. 4,641,557, tools and other devicesusing the preferred reader element standoff feature of the presentsystem will also have greatly enhanced longevity without repair overthese prior art tools. As previously indicated, the measurement systemsin these prior art tools are subject to considerable wear and otherbreakdown due to the foam-biased intimate contact between the readerstrip and the reader element.

Use of the present system to make highly precise angular measurementswith a measurement system having great longevity can be accomplished inat least two ways. One approach is to apply reader strip or calibratedreference member 202 to the column or post about which the angle isbeing measured (e.g., the radial arm saw column about which the armpivots, a miter box cylindrical portion about which a rotating tablepivots, or a miter gauge column about which a miter head pivots) and toconfigure the reading surface of reader element 204 to have a curvaturesubstantially matching the curvature of the corresponding column. Thisapproach could also employ the preferred reader element standoff featureof the present system, as well as the preferred capacitive measurementelectronics of the present system, which account for the increasedlongevity and precision of the present system over the systems disclosedin the previously mentioned patent assigned to Emerson Electric.

Another approach to using the present system to create highly accurateangular measurements is to configure the reader strip or calibratedreference member 202 as a protractor lying in a substantially flat planeand to configure a corresponding reader element 204 having asubstantially planar surface. In such a configuration of the presentsystem, the preferred metalized ground regions 260 and 260a as well assignal regions 262 of the preferred reader strip or calibrated referencemember 202 are preferably oriented radially about the pivot point, andmetalized reader regions 266 are preferably also configured in the samemanner. As with the previously described approach to applying thepresent system to make angular measurements, the preferred readerelement standoff feature and the preferred capacitive electronics of thepresent system could be employed. Alternately, either of theseapproaches could be implemented with optical, magnetic, or resistiveelectronics, either with or without the preferred reader elementstandoff feature of the present system.

In using still another approach with the present system to make angularmeasurements, the present hardware and software may be reconfigured toconvert x and y coordinates to angular coordinates.

The unique combination of mechanical, electronic and software subsystemsin the present system combine to provide much of its flexibility andadaptability to a wide variety of tools and other devices, while at thesame time providing high precision, and substantial longevity at arelatively low cost. As has previously been mentioned, its unique readerelement standoff feature increases longevity and, when used with thepreferred capacitive measurement electronics, provides highly precisemeasurements using a low cost molded reader head and a low cost extrudedguide rail. The preferred wiper springs 122 configuration plays animportant role in providing the substantially nominal reader element 204standoff, while at the same time providing "independent suspension" ateach wiper spring to permit the reader element standoff to vary wheneverperturbations such as those found in extruded guide rail material areencountered. Importantly, the preferred capacitive measurement systemelectronics permit highly accurate measurements in spite of thenominally constant reader element standoff and the permitted variations.This is in contrast to prior art capacitive measurement systemsrequiring substantially intimate contact between the reader strip orcalibrated reference member and the reader element. The preferredconfiguration of wiper springs 122 also maintains a fixed lateralposition of reader element 204 in reader head 100 while permitting thenominally constant standoff between reader element 204 and reader stripor calibrated reference member 202 to occur.

While the precision and longevity of the present system are accomplishedthrough its preferred capacitive measurement electronics and readerelement standoff subsystems, the flexibility of the system to easilyprovide metric, English, decimal and fractional units, to provide bothlinear and angular system readouts, to provide a bidirectional directread measurement (such as of right-of-blade and left-of-blade) and toprovide the preferred ranges of resolution, are all the result of alsoincorporating the preferred software subsystem. This unique combinationcan be tailored to meet the needs of virtually any industry. Forexample, through easily made modifications, the preferred softwaresubsystem can provide measurements in yards for use by the textile andclothing industries.

Accordingly, the present system provides a measurement system havinghardware means comprising hardware for configuring the physicalstructure of the measurement system; electronic means comprisingelectronics for providing a direct display readout of the measurementwithout the need for interpretation of a mechanical analog scale andreader element; and software means comprising software for convertingthe display to desired units of measure without the need for humanmeasurement conversion.

It will be readily apparent to those skilled in the art that manymodifications to the preferred embodiment of the present invention arepossible without deviating from the scope and spirit of the presentinvention. For instance, it will be readily understood by those skilledin the art upon reading and understanding this specification anddrawings that the control structure of the present invention may beimplemented in a variety of different ways, including the use ofdifferent microprocessor control, ROM microcode control, PLA or PALlogic structures and other types of hard-wired or software-controlledmachines.

Thus, while the present invention has been described in connection withpreferred embodiments thereof, it will be understood that manymodifications will be readily apparent to those of ordinary skill in theart. Therefore, it is intended that this invention be limited only bythe claims and the equivalents thereof.

What is claimed is:
 1. A capacitive measurement system, comprising:(a)means for generating a plurality of waveforms; (b) reader referencemeans comprising a calibrated reference member for use with a readerhead in making a measurement, the calibrated reference member comprisingone element of a capacitor; (c) reader head means comprising a readerelement configured to move laterally with respect to the calibratedreference member for making the measurement relative to a referenceposition on the reader reference means, the reader elementcomprising:(i) a plurality of reader regions comprising the otherelement of the capacitor, the reader regions further comprising meansfor receiving the waveforms; and (ii) signal input means for receivingtwo signals resulting from coupling of the two capacitor elements, thesignal input means being isolated from the reader regions; and (d) phaseshift detection means for detecting a predetermined phase shift levelbetween the two signals received by the signal input means when there isno lateral movement between the calibrated reference member and thereader element, for detecting a decreasing phase shift level between thetwo signals received by the signal input means when there is lateralmovement in a first direction, and for detecting an increasing phaseshift level between the two signals received by the signal input meanswhen there is lateral movement in the opposite direction, the phaseshift detection means comprising means for detecting the magnitude ofthe lateral movement by detecting the magnitude of the phase shift in aparticular direction.
 2. The system of claim 1 wherein the calibratedreference member further comprises:(a) a length of substrate materialhaving opposite surfaces; and (b) a metalized pattern carried by one ofthe substrate surfaces, the metalized pattern comprising:(i) a metalizedground strip; (ii) a plurality of metalized ground regions having apredetermined width and parallel sides oriented perpendicular to thelength of the metalized ground strip, the metalized ground regions beingspaced a predetermined distance from one another and conductivelycoupled to the metalized ground strip; and (iii) a plurality ofmetalized signal regions alternately located between the metalizedground regions, the metalized signal regions having a predeterminedwidth and parallel sides oriented perpendicular to the length of themetalized ground strip, the metalized signal regions being spaced apredetermined distance from one another and from the metalized groundregions, the metalized signal regions further being electricallyisolated, both from one another and from the metalized ground strip. 3.The system of claim 2 wherein the calibrated reference member furthercomprises:(a) a layer of pressure sensitive adhesive on the othersubstrate surface; and (b) a removable protective covering over thelayer of pressure sensitive adhesive for protecting the adhesive untilthe calibrated reference member is to be installed.
 4. The system ofclaim 2 wherein the metalized pattern comprises nickel-plated copper. 5.The system of claim 4 wherein a protective dielectric layer covers themetalized ground and signal regions.
 6. The system of claim 1 whereinthe reader regions comprise metalized regions having a predeterminedwidth and parallel sides, the metalized reader regions being spaced apredetermined distance from one another, the metalized reader regionsfurther being electrically isolated from one another.
 7. The system ofclaim 1 wherein the reader element comprises:(a) a first group ofmetalized reader regions each having a predetermined width and parallelsides, the reader regions within the first group being spaced apredetermined distance from one another and comprising means forreceiving in common one of the plurality of waveforms; and (b) a secondgroup of metalized reader regions each having a predetermined width andparallel sides, the reader regions within the second group being spaceda predetermined distance from one another and comprising means forreceiving in common another of the plurality of waveforms.
 8. The systemof claim 1 wherein the reader head means comprises means forestablishing the location of a physical structure by referencing off ofeither of two opposing sides of the physical structure.
 9. The system ofclaim 1 wherein the reader head means comprises display means fordisplaying the measurement in decimal units.
 10. The system of claim 1wherein the reader head means comprises display means for displaying themeasurement in fractional units.
 11. The system of claim 1 wherein thereader head means comprises display means for displaying the measurementin both decimal and fractional units.
 12. The system of claim 1 whereinthe reader head means comprises display means for displaying themeasurement in fractional units having the lowest common denominator.13. The system of claim 1 wherein the reader head means comprisesdisplay means comprising:(a) first mode display means for increasing themeasurement as the reader element moves away from a first side of thereference position and for decreasing the measurement as the readerelement moves toward the first side of the reference position; and (b)second mode display means for increasing the measurement as the readerelement moves away from the other side of the reference position and fordecreasing the measurement as the reader element moves toward the otherside of the reference position.
 14. The system of claim 1 wherein thereader head means comprises display means for displaying the measurementin English units.
 15. The system of claim 1 wherein the reader headmeans comprises display means for displaying the measurement in metricunits.
 16. The system of claim 1 wherein the reader head means comprisesdisplay means for displaying the measurement in both English and metricunits.
 17. The system of claim 1 wherein the reader head means comprisesdisplay means for establishing a zero reading at any location of thereader head means.
 18. The system of claim 1 wherein the reader headmeans comprises display means for providing the measurement in either oftwo ranges of precision.
 19. The system of claim 1 wherein the readerhead means comprises display means for displaying the measurement, thedisplay means comprising coupling means for connecting the display meansto the reader head means so that the display means can be readhorizontally independent of whether the lateral movement of the readerhead means is oriented horizontally, vertically, or in some otherdirection.
 20. The system of claim 19 wherein the coupling meanscomprises means for remotely locating the display means from the readerhead means.
 21. The system of claim 1 wherein the reader reference meanscomprises means for- making an angular measurement.
 22. The system of.claim 1 further comprising magnet means comprising a magnet forcoupling the reader head means to a physical structure by magneticattraction between the magnet, the reader head means, and the physicalstructure.
 23. The system of claim 1 wherein the reader reference meansfurther comprises guide rail means, comprising:(a) a surface foraffixing the calibrated reference member; (b) means for guiding thelateral movement of the reader head means along the calibrated referencemember.
 24. The system of claim 23 wherein the guide rail meanscomprises a Delta International Machinery Corp. Unifence®saw guide frontguide rail.
 25. The system of claim 23 wherein the guide rail meanscomprises a guide rail configured for retrofitting the measurementsystem onto a tool or other device.
 26. The system of claim 1 whereinthe reader head means comprises wiper means comprising:(a) first andsecond wiper elements; and (b) bias means for biasing the wiper elementsinto wiping contact with the calibrated reference member.
 27. The systemof claim 26 wherein the wiper elements each comprise means for securingthe reader element into a fixed lateral position within the reader headmeans.
 28. The system of claim 27 wherein the wiper elements eachcomprise side loading means for exerting a lateral pressure on interiorsidewalls of the reader head means.
 29. The system of claim 28 whereinthe side loading means each comprise a leaf spring.
 30. The system ofclaim 26 wherein the wiper elements each comprise standoff means forcooperating with the vertical stops in order to maintain a nominallyconstant standoff distance between the reader element and the calibratedreference member.